Magnetic recording media generally comprise a magnetizable layer coated on at least one side of a nonmagnetizable support. For particulate magnetic recording media, the magnetizable layer comprises a magnetic pigment dispersed in a polymeric binder. The magnetizable layer may also include other ingredients such as lubricants, abrasives, thermal stabilizers, antioxidants, dispersants, wetting agents, antistatic agents, fungicides, bactericides, surfactants, coating aids, and the like.
Some forms of magnetic recording media, e.g., magnetic recording tape, also have a backside coating applied to the other side of the nonmagnetizable support in order to improve the durability, conductivity, and tracking characteristics of the media. The backside coating typically comprises a polymeric binder and other ingredients such as lubricants, abrasives, thermal stabilizers, antioxidants, dispersants, wetting agents, antistatic agents, fungicides, bactericides, surfactants, coating aids, and the like.
Chromium dioxide-based magnetic recording media (hereinafter "CrO.sub.2 media") are media in which the magnetic pigment of the magnetizable layer comprises chromium dioxide magnetic particles. Environmental risks associated with chromium dioxide have been a serious roadblock obstructing the commercial success of CrO.sub.2 media. In the presence of water, the chromium of chromium dioxide tends to be emitted in both the highly carcinogenic hexavalent state, Cr(VI), and the relatively low toxicity trivalent state, Cr(III). One typical decomposition scheme may be represented by the following reaction: EQU 3CrO.sub.2 +H.sub.2 O.fwdarw.CrO.sub.3 +2CrO(OH).
It has been determined that highly toxic Cr(VI) can leach from CrO.sub.2 media under testing conditions specified by the United States Environmental Protection Agency ("EPA"). As a result, scrap, used, spent, waste, or by-product CrO.sub.2 media are classified as a characteristic hazardous waste by the EPA, and the disposal of CrO.sub.2 media is carefully regulated. Disposing of scrap, used, spent, waste, or by-product CrO.sub.2 media, therefore, can be expensive, can contaminate the environment, and can expose the owner of the media to potential future liabilities under environmental laws and regulations.
One approach for disposing of CrO.sub.2 media involves incinerating the media. During incineration, media containing CrO.sub.2 particles is burned at high temperatures along with vast quantities of other waste materials. The result of such incineration is an ash contaminated with a variety of various incinerated materials. Due to its impure nature, the ash is generally useless as a recycled raw material and commonly ends up in a landfill.
The combustion chemistry of chromium oxides such as CrO.sub.2 upon incineration is not well understood. Although Cr(VI) could be expected to result from combustion, the EPA has speculated that most of the chromium may be emitted as Cr(III) given that Cr(VI) is so reactive. Even so, the EPA is unable to conclude that Cr(VI) emissions from incineration would always represent less than 10% of total chromium emissions. The EPA assumed at one time that Cr(VI) emissions represent 10% of total chromium emissions upon combustion of chromium oxides. See EPA Technical Background Document: Controls of Metals and HCl Emissions from Hazardous Waste Incinerators, page 68 (August 1989). More recently, the EPA has concluded that 100% of chromium emitted from boilers, industrial furnaces, and incinerators is in the hexavalent state. See 56 Fed. Reg. 7165 (Feb. 21, 1991).
U.S. Pat. No. 4,099,949 teaches that chromium (III) oxide is oxidized to CrO.sub.2 and CrO.sub.3 in the presence of air, but that the higher oxides are reduced back to chromium (III) oxide by metal electrodes in an electric furnace (col. 3, lines 3-17). This reaction mechanism ruins the electrodes (col. 3, lines 18-23). To protect the electrodes, carbon or a carbonaceous material may be used to minimize oxidation or to reduce the higher oxides back to chromium (III) oxide (col. 5, lines 7-15 and 51-52).
Chromium (III) oxide, also known as chromic oxide, chromia, chromium sequioxide, or green cinnabar, has the formula Cr.sub.2 O.sub.3. Chromium (III) oxide, characterized by a bright green color, has a variety of industrial uses such as green paint pigment, a ceramic, a catalyst in organic synthesis, green granules for roofing shingles, component of refractory brick, and an abrasive. The various chromium oxides, including chromium (III) oxide have been described, for example, in Kirk-Othmer Encyclopedia of Chemical Technology, volume 6, pages 82-120 (3d ed. 1979).
What is needed in the art is a technique for recycling CrO.sub.2 media into a recovered useful raw material so that such media do not have to be incinerated and then buried in a landfill.